Dam failure would put 1000 at risk: analysis

The dam break analysis of the proposed Ruataniwha Water Storage Scheme suggests half of Waipawa's 2000 population would be put at risk if the dam was to fail in an earthquake.

Failure of the main dam after construction, assuming the reservoir is full, would likely result in significant damage to infrastructure including bridges, roads, stopbanks and sewage treatment plants, and environmental damage to the river corridor and surrounding flood plain.

The resulting flood wave from a dam break would overtop the stopbanks at the town of Waipawa, and water depths in the area of Bibby Street near the Waipawa sewage treatment works would likely be in the order of two to four metres.

Due to the potential risks, along with the size of the main dam, the potential impact category of the main dam was classified as high, the report by the Hawke's Bay Regional Council said.

"Accordingly, the dam is designed to reflect this standard."

Hawke's Bay Regional Investment Company Limited managing director Andrew Newman said the dam break analysis was hypothetical and separate to the probability of a dam failure occurring.

"The water storage reservoir will be designed and constructed in a way that means the likelihood of failure is small. The safety and wellbeing of the community is paramount and is always at the forefront of our planning."

Former Central Hawke's Bay mayor Trish Giddens said it was fair to say there was a lot of interest in the dam around the rural district.

"But I would have to say people have not considered deeply, what would happen to the dam if there was an earthquake or failure."

Ms Giddens was manager of Anglican Care's Services to Older People and was connected to a number of other community organisations around Hawke's Bay. "There will be a lot of people who will want to submit that this is positive because of the possible economic outcomes for Central Hawke's Bay, as long as any risks are mitigated for the dam."

The dam break analysis report said if the 83 metre high dam wall were to collapse, it would send 90 million cubic metres of water towards Waipawa, potentially causing major damage to the town's infrastructure and putting half of its population of 2000 at risk.

The report stated there would likely be a minimum of two to three hours warning time between the time of start of failure, and the time when the population and infrastructure of Waipawa and Waipukurau would be at risk.

The analysis was finalised in May 2013. Its high potential impact category rating prompted the inclusion of multiple features in the dam's design, which would prevent or minimise the probability of a catastrophic failure. The report was prepared by regional council engineers and peer reviewed by Tonkin and Taylor Ltd.